"Visualizing chemical reactions and light-matter interactions with nanometer-scale resolution" - In 1833, Faraday combined silver and sulfur and discovered the first material with a negative temperature coefficient of resistance, silver sulfide. At the time, the word semiconductor did not even exist. Yet we now know that this first semiconducting material laid the foundation for an entirely new and extremely important class of electronic materials. Today, a similar revolution is unfolding for optical materials. Textbook conceptions of light-matter interactions, such as the notions of exclusively positive refractive indices and reciprocal light propagation, are being redefined by new optical materials. These materials allow light to be controlled in ways previously thought impossible, providing techniques to circumvent the diffraction limit of light and tune both electric and magnetic light-matter interactions. In this presentation, I will describe my group’s efforts to develop such new optical materials, and use them to directly visualize, probe, and control nanoscale systems and phenomena – particularly those relevant to energy and biology. We first explore the optical resonances of individual metallic nanoparticles as they transition from a classical to a quantum-influenced regime. We then use these results to monitor heterogeneous catalytic reactions on individual nanoparticles. Subsequently, using real-time manipulation of plasmonic nanoparticles, we investigate the effects of classical-coupling and quantum tunneling between metallic particles on their optical resonances. By utilizing these effects, we demonstrate the colloidal synthesis of an isotropic metafluid or “metamaterial paint” that exhibits a strong magnetic response – and the potential for negative refractive indices – at visible frequencies. Finally, we introduce a new technique, cathodoluminescence tomography, that enables three-dimensional visualization of light-matter interactions with nanoscale spatial and spectral resolution.